Multilayered ceramic structures may be used to form electronic devices such as ceramic capacitors, multilayered ceramic integrated circuits (MCIC), multichip modules, integrated circuit packaging, high temperature sensors (such as exhaust gas sensors), fuel cells, and fuel cell reformer systems. Multilayered structures also find application in transmit/receive modules in phased array radars. These devices may be used as substrates to support and interconnect electronic components mounted thereon, and, to this end, may include open cavities on one or more surfaces for receiving the electronic components.
Such multilayered structures are often made by laminating together layers or sheets of unfired ceramic tape, known in the art as “green-tape,” and then firing the laminated layers to form a finished structure. Green tape is commercially available, for example, from the DuPont Company under the product designation #951AT. The tape contains a material formulation which can be a mixture of glass and ceramic fillers which sinter at about 850° C., and exhibits thermal expansion similar to alumina. Low-temperature processing permits the use of air-fired resistors and precious metal thick film conductors such as gold, silver, or their alloys.
In electronic applications, one or more of the green-tape layers may include metallized portions to provide conduction pathways for electrical current in the finished multilayered structure. The green-tape layers may also have portions punched out to define vias, channels, or cavities. A method of forming cavities in a multilayer LTCC substrate is disclosed, for example, in U.S. Pat. No. 5,855,803, entitled “Template Type Cavity-Formation Device for Low Temperature Cofired Ceramic (LTCC) Sheets” which patent is hereby incorporated by reference.
A conventional method of forming cavities and other openings in a LTCC is by routing or laser cutting openings in each of a plurality of sheets of green tape then stacking and laminating the sheets to form a finished product. FIG. 1 illustrates an overview of the steps involved in a conventional LTCC forming process. At a first step 200, sheets of ceramic tape are blanked, or cut to size from a roll of “green” or unfired ceramic tape. The tape sheets are then stabilized, if necessary, and punched at a step 202 to form vias or other openings that will provide interconnection between the layers when filled with a conductive paste at step 204, often using a printing process that uses a stencil. At step 206, a screen printing process is used to print conductor patterns on the tape sheets. These initial steps 200, 202, 204 and 206 are referred to hereinafter jointly as initial processing steps 208.
Next, various cutting, stacking and laminating steps, designated generally by block 212 in FIG. 1, are carried out. These steps involve cutting openings in the individual sheets using, for example, a routing, laser cutting, or template cutting process. The sheets are then stacked on tooling pins in a given order to form a panel and laminated in an isostatic bath at a pressure of about 3000 psi to make the sheets or layers mold together into a single dense unit.
After the intermediate steps of block 212, the laminated LTCC structure is placed on a router, and the tooling holes are cut away at a step 214. Next, the panel is placed in a furnace and fired at a step 216 to form a fired ceramic panel. The panel is diced into individual parts at step 218.
Steps 212, 214, 216, 218 may be referred to together hereinafter as finishing steps 220. The initial processing steps 208 and finishing steps 220 are conventional and will not be discussed in detail hereinafter. It should also be noted, however, that the initial processing steps 208 and the finishing steps 220 are illustrative only, and, while they are generally useful in the formation of LTCC structures, they can be changed without affecting the intermediate steps discussed hereinafter in connection with various embodiments of the present invention.
The above described method cannot easily be used to form an LTCC having slots or sockets because the single tape sheets cannot withstand the pressure of the isostatic bath. The material covering the slot will collapse into the opening during final lamination and destroy the product. Therefore, different combinations of intermediate steps have been considered for providing improved LTCC products.
One series of intermediate steps used to form an LTCC structure is described in co-pending U.S. patent application Ser. No. 10/718,805, filed Nov. 23, 2003, assigned to the assignee of the present application and the contents of which are hereby incorporated by reference. FIG. 2 illustrates a simplified version of the process disclosed in that copending application wherein first, second, third and fourth stacks of three sheets of green tape are stacked at step 230. (Each stack includes three sheets to simplify the drawings and discussion. Different numbers of sheets are discussed in the above-referenced application.) The stacks of sheets are then pre-laminated or tack laminated at step 232 in order to form first, second, third and fourth substructures. Some the substructures are routed or otherwise processed to form openings therein at step 234, and the four substructures are stacked and undergo a final lamination step at step 236, before the aforementioned finishing steps 220 are performed.
While useful products are produced by this method, it also produces a large percentage of defective products. Yield using this method has been found to be less than 50 percent. It would therefore be desirable to find a method for producing LTCC products having slots, sockets and/or cavities that had a lower defect rate than processes such as the one described above.